correct python markup

Usecases/README.md

@@ -36,7 +36,7 @@ To understand how usecases are set up, let's have a look at the file
 Each script is broken into certain parts, which we explain in what follows.
 
 ### PREREQUISITS
-~~~python3
+~~~python
 # PREREQUISITS  ###############################################################
 # check if output directory "./output" exists. This will be used in
 # the generation of the output string.